Dredged material doing good

It is often possible to use dredged material from ports and harbours to create areas of benefit to wildlife or people. Dr Jez Spearman FIMarEST, a technical director at HR Wallingford, explains some of the ways sediment is contributing to nature based solutions and how he has been developing computer models that show the impact of Harwich initiatives. 

Many of the ports and harbours in the UK and around the world lie adjacent to ecologically important areas of coastal habitat. Port authorities, coastal managers and regulators regularly have to meet both the demands of making ports more efficient by digging bigger channels and berths to accommodate larger ships, while preserving those coastal habitats.  

There is an increasing recognition that dredged material is a resource, rather than a waste, and we are seeing various ways that it can be part of nature-based solutions. At HR Wallingford, for instance, we’ve worked on projects where dredged sediment has been used to create new habitats and coastal defences, to nourish beaches, and to protect salt marshes. These initiatives often also have the added benefit of providing recreational areas for people to use. 

Recycling the material close to shore has other advantages too. It can reduce the amount of carbon emissions emitted by dredgers, as they have to travel a shorter distance. Often, as at Harwich, they have to transport material further offshore. Travelling a short distance in turn reduces dredging costs.   

For the rest of this blog, I’m going to describe how we’ve worked with Harwich Haven Authority (HHA) on using dredged sediment to create new habitats. HHA is the guardian of the Stour and Orwell estuary system in the south-east of the UK, which is where the busy port of Felixstowe is located. There's a lot of environmentally sensitive coastal habitat with extensive mudflats and salt marshes. HHA is responsible for maintaining this, including the dredging of the navigation channels to Felixstowe and other smaller berths and walls.  

Since about 1998 HHA has been carefully releasing some of its muddy dredged sediment so that it is taken up into the Stour and Orwell estuary on the flood tide. The aim is to increase the amount of mud present in those estuaries, making them more resilient to erosion.  

This sediment recycling introduces mud into the water column and lets the currents take it away so that it deposits on mudflats. The hope is that, by adding it at the right point, this replicates the natural process of sediment movement in an estuary. 

The benefits of this sediment recycling are really important, as they shore up the feeding grounds of birds, but they're really hard to measure. They might only add centimetres, or even less, which has meant it is always been quite difficult to demonstrate the success of sediment recycling. 

We addressed the problem by developing a really detailed numerical model of the way sediment enters and moves around within the estuary system. This allowed us to identify what happens to the material that is released in the sediment recycling. We found that about 20% found its way onto the intertidal or very shallow sub-tidal areas, and this sedimentation clearly explained the features observed in surveys of the estuary bed. We were able to compute that the sediment recycling contributed nearly two hectares of extra mudflat per year. 

While this is a great initiative, HHA sought to do more, as only a small amount of its dredged material was being used in this sediment recycling. The rest was still transported offshore. 

HHA therefore developed a new type of dredger, called the Tiamat. Not only is it pretty small and cheap to run, but instead of taking sediment a long way offshore, it releases the sediment into the water above it, for the currents to either transport away or to bring further into the estuary. The Tiamat provides similar benefits to the sediment recycling, because all this segment goes into the water to be taken upstream on the flood tide, but it's cheaper and has a lower carbon footprint.  

We worked with HHA to evaluate the new dredger, using our detailed numerical model. We found that the small dredger was actually as good as a much larger, expensive dredger at removing sediment, because it can spend its time dredging, rather than have to transport the material offshore. 

These sorts of nature based solutions are exciting as they provide environmental benefits while often also being cost effective. It’s important for ports to consider early on in the planning process what they may be able to achieve environmentally with their dredged material. This will make sure it is in incorporated into the project plan and at the contracting stage. Extra studies may be required, but we believe it’s definitely worth the effort.